The present invention relates to a wireless communication method that uses OFDM (Orthogonal Frequency Division Multiplex) for wireless communication, and more particularly to a method for implementing cellular communication.
The research and development of a wireless communication method using OFDM is under way to increase the speed and capacity of wireless communication. In OFDM, transmission data is generated in the frequency domain, converted to time domain signals via IFFT (Inverse Fast Fourier Transform), and transmitted as radio signals. On the receiving side, the time domain signals are converted back to the frequency domain signals via FFT (Fast Fourier Transform) to produce the original information.
During communication, the transmission power of terminals must be controlled on an upstream line, via which signals are transmitted from terminals to base stations, to control the interference power that affects base stations.
The standards-setting organization IEEE802.20 proposes an OFDM-based wireless communication method. IEEE C802.20-06/04 defines an upstream transmission power control method for the wireless communication method described above.
The standards-setting organization 3GPP proposes an OFDM-based wireless communication method as LTE (Long Term Evolution). 3GPP TR 25.814 V7.0.0 (2006-06) defines an upstream transmission power control method for the wireless communication method described above.
The standards-setting organization 3GPP2 proposes an OFDM-based wireless communication method as UMB (Ultra Mobile Broadband). 3GPP2 C30-20060731-040R4 defines an upstream transmission power control method for the wireless communication method described above.
According to the upstream transmission power control of a terminal defined by IEEE802.20 and UMB, a base station decides the transmission power and sends an instruction to a terminal so that a predetermined reception power is given to a channel (there are multiple types) provided for transmitting a specific control signal. On the other hand, for OFDM data channels for transmitting information not belonging to the specific control signal described above such as user data or voices, the terminal increases or decreases T2P (Traffic-to-Pilot) gain according to the index OSI (Other Sector Interference), which indicates the interference status of each sector, to adjust the transmission power of the OFDM signal. In the description above, a sector refers to the logical division unit of base stations via a beam, and a terminal communicates directly with a sector. A T2P gain, which refers to the magnitude of the OFDM data channel transmission power with respect to the pilot transmission power, is defined by the transmission power per OFDM sub-carrier, that is, the power spectrum density.
First, each sector measures the interference power and the thermal noise power and, based on the measured result, calculates IoT (Interference over Thermal). IoT refers to a ratio of the interference power, which is received by a sector from the terminals whose RLSS (Reverse Link Serving Sector) is not the sector itself, to the noise power. An RLSS refers to a sector to which a terminal is to transmit data via the upstream line.
Each sector determines the interference status (0, 1, or 2) based on the calculated IoT and notifies this status to a terminal as the OSI. OSI=0 indicates low interference, OSI=1 indicates high interference, and OSI=2 indicates very high interference.
The OSI is notified from the sector to a terminal via F-OSICH (Forward OSI channel) or F-FOSICH (Forward Fast OSI Channel).
The terminal detects the OSI transmitted from the sectors defined by OSIMonitorSet and performs the operation according to a policy that the T2P gain is increased when the OSI is 0 and is decreased when the OSI is 1 or 2. OSIMonitorSet refers to a set of neighboring sectors, except the RLSS, that is pre-defined by the terminal.
More specifically, when the OSI value is 0, the terminal calculates the probability at which the power is increased and, based on the probability, decides whether the power is increased or not changed. When the OSI value is 1, the terminal calculates the probability at which the power is decreased and, based on the probability, decides whether the power is decreased or not changed. The probability at which the power is increased or decreased is calculated based on the current transmission power of the terminal and the magnitude of contribution to the interference in the base station. When the OSI is 2, the terminal always decides that the power be lowered. Whether the power is increased, decreased, or not changed corresponds, respectively, to three decision values.
After calculating the decision value described above for each sector belonging to OSIMonitorSet, the terminal calculates the average by weighting the decision values with propagation attenuations from each sector to the terminal so that the contribution of a nearer sector becomes larger.
Let the calculated value be Dw. If Dw is equal to or smaller than a threshold, the terminal decreases the T2P gain by a predetermined value. If Dw is equal to or larger than another threshold, the terminal increases the T2P gain by a predetermined value. If Dw does not satisfy either condition, the terminal does not change the T2P gain. The operation described above controls the transmission power of a terminal so that the transmission power per sub-carrier of a terminal near the center of a cell is increased and the transmission power per sub-carrier of a terminal distant from the center of a cell is decreased.